Evolutionary biology

No argie-bargie

Immigrant ants in Europe are too polite to each other for evolutionary theorists

ANTS are famously collaborative. But they are also fiercely competitive. Workers from the same colony sacrifice their own reproductive opportunities to boost that of a single queen or, occasionally, a small ruling council. But they defend the patch on which their nest sits vigorously—often to the death. Unless, that is, they are Argentine ants in Europe.

It has been known for some time that this species, although normally aggressive in its native habitat, has relaxed into placid mutual toleration in the laid-back environment of the Mediterranean. But until Tatiana Giraud, of the University of Lausanne, and her colleagues started probing, it was not realised just how far this toleration extended.

Dr Giraud, whose work has just been published in the Proceedings of the National Academy of Science, collected workers from 33 Argentine-ant nests scattered along the Mediterranean coasts of Italy, France and Spain, and the Atlantic coasts of Spain and Portugal. Measured along the coastline, the extremities of this distribution are 6,000km apart.

Ants from different nests were then paired off to see if they would tolerate each other, or fight. Some 30 of the nests proved to be mutually tolerant. The other three (all from the coast of Catalonia, in Spain) were also mutually tolerant, but they fought to the death with workers from the larger group. Argentine ants in Europe seem, therefore, to belong to two separate but widespread “supercolonies”, the larger of which is the biggest collaborative society in the known living world.

Such chumminess is quite disturbing to evolutionary biologists. The willingness of worker ants to delegate reproduction to their queens was hard enough to explain until, in the 1960s, a biologist called William Hamilton came up with the idea of “kin selection”. This showed that, besides the usual way of passing genes to the next generation by mating and reproducing, an individual can increase its genetic footprint in the future by helping close relatives to raise young, so long as the cost of doing so is not too high.

That helps to explain human nepotism. But it applies particularly strongly to ants (and also to bees and wasps), because they have a peculiar form of genetic inheritance in which females (and only females) share more genes with their sisters than with their daughters. Helping your mother (ie, the colony's queen) to raise female offspring thus makes better genetic sense than breeding yourself. That, in turn, explains why the sterile workers in these species are always female.

There is, however, no theory involving kin selection that explains why unrelated neighbouring colonies should collaborate. And there is certainly no reason why unrelated colonies thousands of kilometres apart should do so. The assumption, therefore, had been that these colonies would turn out to be closely related. That might make sense for an introduced species: it could be the result of a single introduction, or rather of two introductions, one for each supercolony. But this assumption turns out to be wrong.

Dr Giraud and her colleagues looked at variations in 17 genetic markers, known as microsatellites, in ants from the various nests. Taken together, these markers form an index of how closely related the worker ants from the various nests are. The evidence suggests that Argentine ants in Europe are the result of between six and 13 unrelated pregnant queens having been introduced into the continent. Too much variety, in other words, to explain the toleration that they show to one another.

The researchers have come up with a different hypothesis, of sorts. This is that the introduced ants, now liberated from the parasites and competitors of their native land, have become abnormally abundant. In these circumstances, aggression towards others of their species might become a positive liability if it caused them to spend too much time fighting and not enough gathering food for the nest. Dr Giraud proposes that natural selection might cause convergence in the genetic characteristics by which ants tell relatives from aliens, so as to reduce the amount of fighting. But since nobody knows precisely what those characteristics are, it is impossible to test this hypothesis.

The team does make one prediction, however: that the supercolonies will prove evolutionarily unstable. Since neighbouring nests often exchange workers, there is ample scope for queens to free-ride on the labour of unrelated individuals. In the harsh Darwinian world, such genetic mugs cannot last forever. If and when a mutation arises that stops them giving something for nothing, it should spread rapidly. The Argentine ant in Europe could thus provide an interesting test of natural selection in action.